Nearly all of modern tax theory is concerned with what actions, or states of affair, trigger tax liability, and virtually none is concerned with the remittance of funds to the government to cover that liability. Indeed, elementary public finance textbooks assert that the remittance details - such as whether the buyer or seller of a commodity remits the sales tax triggered by the sale - are irrelevant to the consequences of a tax. Operation of actual tax systems, however, requires considerable attention to the remittance of monies to the tax authority. This attention includes both the administration and enforcement of the tax rules, and the design of the tax rules with the administrative and compliance issues in mind. This chapter will address, from a theoretical and empirical perspective, the practical issues that arise in the operation of a modern tax system, with particular attention to the UK tax system.